This invention pertains to the use of epoxy resins in adhesive compositions with improved properties. The invention also provides for a physically stable dispersion which may be used in such adhesive compositions.
Epoxy resins have a spectrum of properties that are well adapted for use in coatings, adhesives, fiber-reinforced laminates, composites, engineering plastics and specialty applications, such as potting resins and mortars. Such properties include excellent strength, toughness, corrosion and solvent resistance, good adhesion and electrical properties, good dimensional stability, hardness, and low shrinkage on cure.
Adhesives are a particularly important application for epoxy resins. Epoxy-based adhesives offer many advantages to end-users such as high strength and moisture resistance while providing protective seals as well as load-bearing properties.
Generally, epoxy resins are formulated into either one- or two-component, ambient or heat-cured paste or film adhesives. Typical ingredients, in addition to the epoxy resin, include curing agents, catalysts and accelerators, fillers, pigments, reactive diluents, nonreactive diluents, solvents, flexibilizers, toughening agents, extenders and rheological control agents.
Good rheological control is important to an epoxy adhesive. It must thin under shear to allow for easy application and wetting of surfaces; however, once it has been applied slumping or sagging of the adhesive bead can be a problem. At the same time, separation of the formulation ingredients must be avoided.
Heretofore, adhesive formulations based upon epoxy resins, toughened with insoluble dispersions of acrylic elastomers, show poor rheological control. Epoxy resins toughened with reactive liquid polymers, such as carboxyl-terminated butadiene-acrylonitrile liquid polymers, also pose certain difficulties in formulating and handling.
First, resins in the prior art toughened with reactive liquid polymers need a fair amount of fumed silica to achieve good rheological control. High loading of fumed silica, usually more than 5 weight percent, is necessary to achieve good rheological control. High concentrations of fumed silica or other fillers may cause abrasion problems, alter polymer properties and lower the adhesive strength. Second, since these resins are highly viscous (500,000 to 900,000 centipoise (cps) at 25.degree. C.), it is difficult to incorporate large amounts of fumed silica into them. Therefore, additional diluents and processing steps, such as a let-down procedure, are required for the formulation of such epoxy resins toughened with reactive liquid polymers.
Rheological control in paste-type adhesive systems is especially important in robotic dispensing applications. In automotive assembly lines the adhesive is dispensed with a robot and the part with uncured adhesive is transferred to other work stations for curing and further processing. Under these conditions it is imperative that the assembly line is not contaminated with drippings from uncured adhesive.
In a one-part epoxy adhesive, satisfactory rheological control for the uncured system requires ease of pumping and no stringing once the adhesive is dispensed and the robot is removed. Then, regardless of the type of surface (cold-rolled steel, hot-dipped galvanized, electrogalvanized, aluminum, etc.) and regardless of the cleanliness of the surface (solvent-wiped, oily, laser-treated, etc.) the adhesive needs to stay on the surface without sagging, drooping or dripping. Most commonly, one-part epoxy adhesives are cured at elevated temperatures. As the temperature of the adhesive is increased, resin viscosity will decrease and the wettability of the adhesive will increase. It is also important, however, that the adhesive does not sag or drip at elevated temperatures before cure takes place.
In a two-part paste adhesive system, sag control can be partially or completely achieved by room temperature cure. In this case, rheological control is achieved by rapid build-up of microstructure. If rheological control is primarily achieved by microstructure build-up at ambient conditions, a balance between cure rate and dispensing rate must be maintained.
In slow gelling two-part adhesives, until the right amount of microstructure build-up occurs, requirements for rheological control are similar to one-part adhesives. Again, handling, abrasiveness and loss of properties are valid concerns. In addition, long molecular chains, formed because of the ambient cure, will be a likely cause of stringing. In this case, depending on the viscosity of the system, some mechanical solutions can be offered. If the mechanical approach is not adequate or cannot be used, this invention offers solutions for two-part as well as for one-part adhesives.
Four U.S. patents describe the preparation of stable acrylic elastomer dispersions in epoxy resins for toughened epoxy resins: U.S. Pat. Nos. 4,521,490; 4,524,181; 4,708,996; and 4,789,712. While these patents describe a wide range of preferred vinyl monomers and comonomers, in particular the esters of acrylic or methacrylic acid with alkanols having 1 to 18 carbon atoms, the inclusion of hydroxy-functional comonomers for rheological control is not considered.
The formulation of acrylic dispersions into paste adhesives is described in U.S. Pat. No. 4,521,490. However, the question of rheological control is not addressed in this patent. In particular, the formulation of the dispersion with fumed silica and its rheology is not described.